Photooxygenation of olefins, phenol, and sulfide using fullerodendrimer as catalyst

Article abstract of DOI:10.1246/cl.2004.1142

Fullerodendrimer acts as a new catalyst that uses oxygen and light to generate singlet oxygen (¹O₂)- The dendrimer facilitates various types of singlet oxygenation reactions including ene reaction, the Diels-Alder reaction, and oxidation of phenol and sulfide.

Full text of DOI:10.1246/cl.2004.1142

1142
Chemistry Letters Vol.33, No.9 (2004)
Photooxygenation of Olefins, Phenol, and Sulfide Using Fullerodendrimer as Catalyst
Yutaka Takaguchi,Ã Yasushi Yanagimoto, Shohoko Fujima, and Sadao Tsuboi
Graduate School of Natural Science and Technology, Okayama University,
Tsushima-Naka 3-1-1, Okayama 700-8530
(Received May 18, 2004; CL-040565)
Fullerodendrimer acts as a new catalyst that uses oxygen
O
OMe
and light to generate singlet oxygen (1O2). The dendrimer facil-
itates various types of singlet oxygenation reactions including
ene reaction, the Diels–Alder reaction, and oxidation of phenol
and sulfide.
N
O
OMe
O
NH
O
O
N
N
H
NH
Fullerene-based derivatives have received considerable at-
tention because of a variety of interesting features in structural
chemistry and material science.1 In particular, photosensitizers
made of fullerene have been attracting a great interest in the field
of synthetic organic chemistry2–6 and photodynamic therapy.7 In
terms of chemical applications, several reports are found in the
literature on ene3,5,6 and [4+2]cycloaddition4–6 reactions of ole-
fins with 1O2 generated by photosensitization of ground-state
oxygen with fullerenes or fullerene derivatives. Nakamura re-
ported that C60, C70, and their derivatives are synthetically useful
photosensitizers as assessed for the photooxygenation of olefins
with singlet oxygen.6 Lattassa et al. have reported a systematic
study on the selective photooxidation of sulfides to sulfoxides
by the use of a fullerene-based heterogeneous photosensitizer.8
However, chemical uses of 1O2 produced by fullerenes are often
hampered by the limited solubility of the carbon cages in most
solvents or the difficult recovery of soluble fullerene derivatives
at the end of the reaction. Recently we have reported a facile syn-
thesis of fullerodendrimer,9 of which fullerene moiety is proved
to be efficient photosensitizer to generate singlet oxygen.9c De-
spite the many studies focused on fullerodendrimer them-
selves,9,10 very little has been determined about fullerodendrim-
er-mediated chemical transformations. Fullerodendrimer might
be candidate molecule for photocatalyst because of versatile sol-
ubility introduced by dendritic substituent. This paper describes
homogeneous photocatalytic activity of the fullerodendrimer to
generate singlet oxygen.
In order to probe the scope of the fullerodendrimer sensiti-
zation and to investigate the nature of the reactive species so
generated, we subjected a series of typical olefins to photooxy-
genation sensitized by fullerodendrimer (1), which was synthe-
sized as described before.9b The Diels–Alder reaction and the
ene reaction are two widely used applications of singlet oxygen
with olefins.11 Thus, we examined the Diels–Alder conversion of
ꢀ-terpinene (2) to ascaridole (3) (Eq 1), and furan-2-carboxylic
acid (4) to lactone (5) (Eq 2), as well as the ene reaction of 2,3-
dimethyl-2-butene (6) (Eq 3) and 4-methylpent-3-en-2-ol (8)
(Eq 4). Furthermore, oxidation of 1-naphthol (11) (Eq 5) and di-
benzyl sulfide (13) (Eq 6) was also examined. All six reactions
proved to be successfully catalyzed by fullerodendrimer 1 under
the same reaction conditions as used by Nakamura et al.6 The
brown color of dendrimer 1 persisted until the end of the reac-
tion.12
O
N
O
OMe
OMe
1
In typical run, a toluene solution (7 mL) of ꢀ-terpinene 2
(83 mg, 0.61 mmol) in the presence of fullerodendrimer 1
(1 mol %, 9.0 mg, 0.0059 mmol) was irradiated with a 500-W
high-pressure mercury lamp through a Pyrex filter at room tem-
perature. During irradiation, oxygen was passed through the re-
action mixture. After a 1 h irradiation, the mixture was concen-
trated to afford the crude product as colorless oil. Purification by
silica gel column chromatography (10% EtOAc/hexane) gave
ascaridole 3 (99 mg, 0.059 mmol) in 93% yield.13 On the other
hand, photooxygenation of furan-2-carboxylic acid 4 proceeded
employing only 0.1 mol % of dendrimer 1 to give the lactone 5 in
99% yield after 80 min (Eq 2). Furthermore, we confirmed the
reactivity of the fullerodendrimer 1 as the sensitizer by the use
of the ene reaction with 2,3-dimethyl-2-butene 6 as a model re-
action. Compound 6 was easily converted to 3-hydroperoxy-2,3-
dimethyl-1-butene (7) in 97% yield by the use of fulleroden-
drimer 1 (1 mol %) as photocatalyst (Eq 3). Ene reaction of 4-
methylpent-3-en-2-ol 8 with singlet oxygen generated by photo-
sensitization of fullerodendrimer 1 gave compounds 9 and 10 in
98% and 1.9% yields, respectively (Eq 4). The observed regio-
selectivity and the stereoselectivity of the products conform to
the known selectivity of singlet oxygen reactions,6 indicating
that the fullerodendrimer-sensitized reaction generates free sin-
glet oxygen. Meanwhile, oxidation reactions of phenol and sulf-
oxide were also examined. Photooxidation of 1-naphthol 11 in
the presence of dendrimer 1 (1 mol %) gave 70% conversion to
only 1,4-naphthoquinone (12) after 50 min (Eq 5). The perform-
ance of the fullerodendrimer 1 in the photooxidation of sulfides
to sulfoxides was tested on benzyl sulfide (13). Photooxygena-
tion of dibenzyl sulfide 13 sensitized by 1 (1 mol %) afforded
the corresponding sulfoxide (14) in 90% yield (Eq 6). In this
case, a mixture of chloroform and methanol was used as the sol-
vent of choice according to the observation of Foote and Peters
that protic solvents, minimizing the physical quenching of sin-
glet oxygen, afforded better yields of sulfoxides than aprotic
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.9 (2004)
1143
ones.14 Hence, when we used pure chloroform as a solvent of the
photooxidation reaction, sulfoxide 14 was obtained in only 59%
yield. It is remarkable that no side or cleavage reactions were ob-
served with the benzylic substrates, in particular the Pummerer
rearrangement which is often descried in this context. As op-
posed to C60 and many of its derivatives, the fullerodendrimer
can be readily soluble and used in protic solvents such as meth-
anol which makes the photooxygenation highly selective, afford-
ing good yield of sulfoxides with benzylic sulfide.
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3
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(1)
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1 h
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(2)
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(3)
(4)
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OH
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+
86 : 14
OOH
10
4.5 h
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OH
hν , O2
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CHCl3
(5)
(6)
50 min
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13
14
In summary, we have shown that fullerodendrimer-sensi-
tized photoreactions generate free singlet oxygen in synthetical-
ly useful amounts. One simple illustrative advantage is that full-
erodendrimer is soluble in a wide variety of solvents while
maintaining the chemical properties of the crucial C60 core. In
particular, formation of singlet oxygen in protic solvent is quite
useful. Further work is in progress to explore the applications
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This work was partly supported by Research Foundation for
Materials Science, Saneyoshi Scholarship Foundation and the
Ministry of Education, Culture, Sports, Science and Technology
(15750036, 15550036).
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12 Decomposition of fullerodendron 1 was observed by GPC
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13 When we used pristine C60 as photosensitizer under com-
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3 in 75% yield.
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Published on the web (Advance View) August 7, 2004; DOI 10.1246/cl.2004.1142